on the one hand trillian is a desktop-app connecting to multiple other protocols and on the other hand trillian has it's own protocol.
even this protocol is based on jabba, i'm not sure if it's that open so anyone could write an messenger for this protocol as an out-app.
it's possible to use trillian as an in-app, but it would may be more useful to create an icq/msn/etc.-bot.